Preheat oven to 350 degrees F. Generously grease and flour an 8 1/2 x 4 1/2-inch loaf pan.
In a small bowl, dissolve the instant espresso powder in the boiling water. Set aside and allow to cool.
In the bowl of a stand mixer fitted with the paddle attachment, add in the yogurt, cornmeal, butter, sugar, eggs, flour, baking powder, baking soda, salt, and vanilla extract.
Beat on medium speed until all ingredients are well combined, about 1 to 2 minutes.
Divide half the batter into another bowl and add in the dissolved espresso and cocoa powder. Mix until combined.
Spoon the batter into the prepared pan, alternating spoonfuls of the vanilla and mocha batter, creating a checkerboard pattern. Create 3 layers of this pattern. Run a knife or long skewer through the batter, using a swirling motion, to create the marble effect.
Bake for about 55 minutes, or until a skewer inserted comes out clean. Allow to cool in the pan on a cooling rack for 15 minutes, then remove from loaf pan, and cool completely on a cooling rack.